The instructions above are great, save for the slight possibility of a fish swimming into the bottle. You will have to keep the water level lower than the baffle, or find something to obstruct the entrances into the baffle. I use floating plants and place them inside the baffle so my betta cannot swim into it. It also works as a secondary baffle, and my betta loves me for it.
You can also buy a small foam pad/sponge to place in front of your current filter cartridge to reduce the speed of the water flowing out of the filter. You may have to cram it in there, but it won't hurt anything. I actually just threw away my cartridge and use a large sponge filter I cut into halves as my cartridge. It's a bigger surface area for beneficial bacteria and it also reduces the flow.
So I essentially have three baffles: a "replacement" filter cartridge, a bottle wrapped around the outflow, and live plants inside the baffle to keep fishies out.